The present invention relates generally to valves and, more particularly, to a uniquely configured plug and seal assembly which is specifically configured to mitigate the erosive and cavitating effects of flashing water and steam flowing within a fluid control valve.
In power plants, boiler water is typically circulated within a closed loop by a feedpump. The feedpump takes the water from a deaerator and increases the pressure from ambient up to 6000 psi for delivery to a boiler. Because the feedpump operates at a relatively constant rotational speed, a minimum amount of flow through the feedpump is required in order to avoid overheating and cavitation of the feedpump components. However, the boiler feed flow requirements fluctuate regularly throughout the day in proportion to changing electricity production demands placed upon the power plant. When the boiler feed flow requirements are reduced to a level below the minimum required flow through the feedpump necessary to avoid overheating and cavitation of the feedpump, a recirculation system is engaged to direct a portion of the high pressure flow back to the deaerator or to a condenser and then to the feedpump. Ideally, the recirculation system will meter the flow in response to the feedpump requirements such that an optimal flow level is circulated through the feedpump in order to prevent excessively high feedpump operating temperatures and to prevent cavitation of the internal feedpump components.
A recirculation valve is typically utilized in the recirculation system to selectively block and unblock the flow of high pressure water to the deaerator or condenser. The recirculation valve may be configured as a linear displacement valve. Such recirculation valves include plugs that are linearly displaced during normal operation of the valve. Within these valves, the plug is axially slidable within a valve cage. The valve cage defines a multiplicity of tortuous or non-tortuous fluid passageways. Certain linear displacement valves are configured for xe2x80x9cover plug flowxe2x80x9d wherein water flows radially inward into the interior of the valve cage from the exterior thereof, with the water undergoing a pressure drop as a result of the flow through the valve cage. To open the valve, the plug is lifted off of a seat ring which then allows fluid, such as boiler water, to flow from the interior of the valve cage and out of the valve via the unblocked seat ring. Conversely, the movement of the seating surface of the plug into sealed engagement with the complementary seating surface of the seat ring facilitates a closed or shut-off condition for the recirculation valve.
The recirculation valve must be capable of handling water at extreme thermodynamic conditions. For example, within the interior chamber of the valve, the water pressure may range from 2500 to 6000 psig and the temperature may be at 300xc2x0 F. to 500xc2x0 F. At the outlet of the valve, the pressure of the water may range from only xe2x88x9212 to 10 psig with corresponding temperatures of only 110xc2x0 F. to 240xc2x0 F. Thus, in the open position, the plug and seal of the recirculation valve must be capable of withstanding an extreme pressure drop under flashing conditions while simultaneously minimizing erosion and cavitation of the seating surfaces of the plug and seat ring. Cavitation causes xe2x80x9cpittingxe2x80x9d of the metallic surface of the valve plug and seal that may occur during the sudden extreme pressure drop of the passing boiler water. The xe2x80x9cpittingxe2x80x9d of the plug and seal may result in leak paths that may increase in size over time due to erosion. As distinguished from normal evaporation, flashing is the sudden vaporization of the water caused by an instantaneous temperature and/or pressure drop.
In recirculation valves such as those that are utilized in power plants, the fluid flowing from valve inlet to outlet may undergo a pressure drop of up to 5500 psig at a temperature of up to 500xc2x0 F. Such a pressure drop of the boiler water may be characterized as a violently explosive event occurring within the valve. In addition, in the closed position, the valve seat and plug must be capable of blocking flow with no leakage against pressures as high as 6000 psig. Should even a slight amount of leakage occur, flashing steam and water will cut and erode the seating surface of the metallic valve seat and plug with the effectiveness of a saw blade, quickly increasing the leakage path and rendering the valve useless. The constant leakage of the flashing steam and water will further accelerate the erosion damage of the seat ring and the plug. As mentioned above, during periods of high electricity production demands upon the power plant, the boiler feed flow requirements are at a maximum. Excessive leakage in the recirculation valve may prevent the required flow of water from reaching the boiler resulting in the failure of the power plant to meet the electricity demands that may be placed upon it. In extreme cases, excessive leakage of a recirculation valve may necessitate that the power plant be taken off line in order to replace the leaking valve.
The present invention specifically addresses the above-described erosion and cavitation damage problem by providing a plug and seal assembly with unique, complementary configurations specifically adapted to prevent the water exiting the valve cage from directly impinging the seating surface of the seat ring. The present invention alternatively provides a plug and seal assembly having a redundancy of sealing surfaces in order to reduce the risk of leakage. In this regard, the present invention provides a plug and seal assembly capable of neutralizing the erosion of the seating surfaces due to flashing and cavitation. These, and other features of the present invention, will be described below.
The plug and seal assembly of the present invention is adapted for use in a fluid control valve. The fluid control valve includes a valve housing defining an interior chamber and a flow opening configured to fluidly communicate with the interior chamber. The interior chamber receives the fluid therein and the flow opening allows the fluid to escape the interior chamber. In a first embodiment, the plug and seal assembly comprises an annular seat ring and a generally cylindrical plug. The seat ring is at least partially engaged to the valve housing at the flow opening.
The valve housing may also include a sleeve and a valve cage concentrically disposed within the interior chamber. The valve cage is captured between the sleeve and the seat ring. The valve cage defines annular flow passages configured for reducing the pressure of the fluid flowing therethrough from the interior chamber prior to exiting the flow opening. The sleeve has an elongate bore extending axially therethrough. The plug and seal assembly includes the plug. The plug is comprised of a plug body and a plug head. Extending axially from one end of the plug body is a rod which is advanced through the bore within the sleeve of the valve housing. The rod is coupled to an actuator which reciprocally moves the valve plug between a closed position and an open position. The engagement of the plug to the seating surface defined by the seat ring effectively blocks the flow of fluid out of the interior of the valve cage. Fluid flows into the interior chamber and thereafter radially through the valve cage from the exterior to the interior thereof. When the valve plug is moved from its closed position towards its open position, fluid is able to flow downwardly through the seat ring and out of the fluid control valve. The plug head may comprise a plug head of a first embodiment or a plug head of a second embodiment. Likewise, the seat ring may comprise a seat ring of the first embodiment or a seat ring of the second embodiment. The plug head and seat ring of the first embodiment are configured for use in conjunction with each other as are the plug head and seat ring of the second embodiment.
The seat ring of the first embodiment includes first, second, and third sealing discs alternately stacked with respective first, second and third resilient bearing discs. The bearing discs may be formed of an elastomeric material having high resilience such as carbon-fiber/nitrile sheets. The sealing discs may be formed of a metallic material such as stainless steel. The sealing discs provide support for the bearing discs which, unaided by the sealing discs, are incapable of withstanding the rapid expansion of the fluid as it flows along the plug and seat ring and out of the flow opening. The inner annular edges of the first, second and third sealing and bearing discs collectively define a truncated conical sealing surface of a predetermined half-angle. The plug head defines an upper cylindrical section and stepped first, second and third ramps. Each of the first, second and third ramps has a truncated conical shape of equal half-angle. The first, second and third ramps define respective first, second and third shoulders adjacent the major diameters of the ramps. The half-angle of the conical sealing surface is larger than the half-angle of the first, second and third ramps. The direct engagement of the first, second and third shoulders against respective first, second and third sealing discs acts to deform the respective first, second and third bearing to create three fluid-tight seals therebetween. Thus, the configuration of the plug and seal assembly of the first embodiment creates a redundancy of sealing surfaces in order to reduce the risk of leakage of fluid out of the fluid control valve when in the closed position.
The second embodiment of the plug and seal assembly includes a seat ring having first, second, and third sealing plates alternately stacked with respective first, second and third resilient bearing plates. The inner edge of the first sealing plate defines a first cylindrical surface. The first and second bearing plates and the second sealing plate collectively define a second cylindrical surface of a diameter larger than that of the first cylindrical surface. The third sealing and bearing plates and the upper portion of the seat ring collectively define a truncated conical sealing surface having a predetermined half-angle and a major diameter equal to that of the second cylindrical surface. The bearing plates may be formed of an elastomeric material such as carbon-fiber/nitrile sheets. The sealing plates may he formed of a metallic material such as stainless steel.
The plug head of the second embodiment successively defines an upper barrel, a first taper, an intermediate cylindrical barrel and a second taper. The first taper is interposed between the upper and intermediate barrels with the second taper interposed below the intermediate barrel. The half-angle of the second taper is substantially equal to the half-angle of the conical sealing surface. The diameter of the upper cylindrical barrel is smaller than that of the first sealing plate and larger than that of the second bearing plate. By arranging the plug and seat ring in this manner, the engagement of the first taper to the first bearing plate deforms the second bearing plate to create a seal therebetween. Simultaneously, a surface-to-surface seal is created between the second taper and the conical sealing surface collectively formed by the third sealing and bearing plates and the seat ring.